Balazs,
Sorry that you are seeing this issue.
You could install Subclipse using the dropins method. I just tried installing Subclipse 1.8 to MyEclipse 8.6.1 using dropins method and it works at my end.
Shut down MyEclipse. You can extract the downloaded plugin zip file and copy the features and plugins folder to the dropins folder located in the MyEclipse installation directory. Please make sure the features and plugins folders are immediate children of the dropins folder or the plug ins will not be recognized when you start MyEclipse. Once the folders are copied, please start MyEclipse and the plugin will be automatically detected.
We will look into the login issue, but it is high time you move to a newer and stable version of MyEclipse. Versions earlier than MyEclipse 2018 are no longer fully supported. We do try and assist licensed users working with previous versions but no fixes will be made to those releases.
You can install MyEclipse 2019 alongside your existing MyEclipse 8.6 installation and give it a try. Your current license is valid for newer releases as well.
Please let us know if you see any issues with the migration. We will be glad to assist you with the upgrade.
–Swapna
Genuitec Support